Method of digital epilaxy by externally controlled closed-loop feedback

ABSTRACT

A method and apparatus for digital epitaxy. The apparatus includes a pulsed gas delivery assembly that supplies gaseous material to a substrate to form an adsorption layer of the gaseous material on the substrate. Structure is provided for measuring the isothermal desorption spectrum of the growth surface to monitor the active sites which are available for adsorption. The vacuum chamber housing the substrate facilitates evacuation of the gaseous material from the area adjacent the substrate following exposure. In use, digital epitaxy is achieved by exposing a substrate to a pulse of gaseous material to form an adsorption layer of the material on the substrate. The active sites on the substrate are monitored during the formation of the adsorption layer to determine if all the active sites have been filled. Once the active sites have been filled on the growth surface of the substrate, the pulse of gaseous material is terminated. The unreacted portion of the gas pulse is evacuated by continuous pumping. Subsequently, a second pulse is applied when availability of active sites is determined by studying the isothermal desorption spectrum. These steps are repeated until a thin film of sufficient thickness is produced.

This invention was made with Government support under DE-AC05-85OR21400awarded by the U.S. Department of Energy to Martin Marietta EnergySystems, Inc. and the Government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to digital epitaxy of thin films bydeposition from the gaseous phase, and more particularly, to a methodand apparatus for submonolayer controlled, digital growth of epitaxialthin films of covalently bonded materials.

BACKGROUND OF THE INVENTION

Thin filmed materials are often utilized in semiconductors, ceramics,metals and superconductors. For example, thin layer materials areutilized in the production of highly complex silicon integratedcircuits. A number of methods are known for producing thin filmsincluding vacuum evaporation, molecular beam epitaxy, and atomic layerepitaxy (ALE), etc. Additionally, such materials are useful in thedevelopment of electronic devices based upon bandgap engineering.

Particularly, atomic layer epitaxy has been used for growth of compoundsemiconductors and other thin film structures. C. H. L. Goodman and M.V. Pessa, Atomic Layer Epitaxy, Journal of Applied Physics 60(3) , Aug.1, 1986, R65-R81; M. Ozeiki et al., New Approach to the Atomic LayerEpitaxy of GaAs Using Fast Gas Stream, Applied Physics Letters, Vol. 53,Number 16, Oct. 17, 1988, pp. 1509-1511; U.S. Pat. No. 4,058,430 toSuntola et al.. However, such growth has only been achieved withionically bonded compounds. The growth of other than ionically bondedcompounds by ALE has not been successful. Specifically, layer-by-layergrowth of materials from non-elemental sources (gaseous chemicalcompounds) can not be achieved using ALE.

ALE was developed for the growth of ionically bonded materials, such ascompound semiconductors. In its present form, ALE has only successfullybeen used for compound semiconductor growth. ALE achievesmonolayer-by-monolayer growth by chemisorption of one gas phasecomponent onto a saturated surface having a monolayer coverage of theother component. The reaction proceeds until the surface is saturatedwith the one gas phase component. At this point the saturation cover isbelieved to be only a monolayer thick because once the strongchemisorption bonds are saturated, physisorption is too weak to build upfurther layers. The role of the two components is then exchanged and thereaction is repeated. A film of finite thickness is built up byalternating the exposure of the surface to the constituent components.

ALE growth is conducted at growth conditions collectively known as the"ALE processing window", which is arrived at by systematic empiricalstudy. Generally, the only analytical feedback on layer control in ALEis provided by external, remote analysis of the grown films. In otherwords, the article must be removed from the process chamber and place ina separate and distinct testing device. The exposure times of the growthsurface are then readjusted until this trial and error procedureproduces a material that consists of alternating monolayers ofconstituent elements. The procedure for establishing the "ALE processingwindow" for compound semiconductor growth cannot be directly extended tometals or semiconductors with covalent bonding. In order to producelayer-by-layer growth materials from other than ionically bondedmaterials from gaseous molecular sources, a different controllingmechanism must be utilized.

Another disadvantage of existing ALE technology is that ALE is limitedto binary systems that can simultaneously exhibit strong chemisorptionand weak physisorption at temperatures appropriate for thin film growth.This constraint has limited the application of ALE to ionically bondedcompound semiconductors.

Further, the static nature of ALE is determinative of its capabilities.Therefore, only a fully saturated surface satisfies the requirements forcontrolled growth.

A further disadvantage of existing ALE technology is that film growth inALE occurs per cycle of exposure. Frequently one cycle of exposure isreferred to as a pulse. One cycle of exposure produces a complete atomiclayer of material.

Pulsed supersonic jets have been used to provide rapid deposition of athin film upon heated substrates for the preparation of semiconductorsand similar electronic devices. While the pulsed supersonic jet processgrows a large amount of deposits in a short time, there is difficulty incontrolling the process without feedback data.

As demonstrated by the patents to Hall (U.S. Pat. No. 5,009,485), Aspneset al. (U.S. Pat. Nos. 4,931,132 and 4,332,833), Siegmund et al. (U.S.Pat. No. 4,878,755), Keller et al. (U.S. Pat. No. 4,846,920), Hartley(U.S. Pat. No. 4,770,895), Tien (U.S. Pat. No. 4,713,140), Strand et al.(U.S. Pat. No. 4,676,646), and Cole (U.S. Pat. No. 4,582,431), the useof optical methods for monitoring the development of materials is knownin the art.

The disadvantages and limitations of the prior art techniques discussedabove indicate the need for an efficient, versatile, and controllablemethod and apparatus for layer-by-layer production of thin films. Thesubject invention provides such a method and apparatus.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and apparatus fordeposition of thin film materials in discrete submonolayer incrementsincluding metals, insulators, superconductors, semiconductors and alloysthereof.

It is another object of the invention to provide a method and apparatusfor the deposition of thin films of covalently bonded materials andsemiconductor films selected from elements in Group IV of the PeriodicTable.

It is a further object of the invention to provide a method andapparatus having a closed loop feedback system.

It is also another object of the invention to provide a method andapparatus wherein the closed loop feedback system is achieved bymonitoring the isothermal desorption spectrum of the growth surface.

It is another object of the invention to provide a method for real-timein-situ monitoring of the growth of thin films.

It is also a further object of the invention to provide a method andapparatus having thickness control at the monolayer level stemming fromthe digital nature of the process.

These and other objects of the invention are achieved by the instantmethod and apparatus for digital epitaxy. The apparatus includes apulsed gas delivery assembly that supplies gaseous material to asubstrate to form an adsorption layer of the gaseous material on thesubstrate. Means are also provided for measuring the isothermaldesorption spectrum of the growth surface to monitor the active siteswhich are available for adsorption. The vacuum chamber housing thesubstrate facilitates evacuation of the gaseous material from the areaadjacent the substrate following exposure.

In use, digital epitaxy is achieved by exposing a substrate to a pulseof gaseous material to form an adsorption layer of the material on thesubstrate. The active sites on the substrate are monitored during theformation of the adsorption layer to determine if all the active siteshave been filled.

Once the active sites have been filled on the growth surface of thesubstrate, the pulse of gaseous material is terminated. The unreactedportion of the gas pulse is evacuated by continuous pumping.Subsequently, a second pulse is applied when availability of activesites is determined by studying the isothermal desorption spectrum.These steps are repeated until a thin film of sufficient thickness isproduced.

By providing a closed loop system where thin film growth can bemonitored and the growth parameters adjusted in real time, and byproviding a system whereby thin films of covalently bonded materials canbe produced, the problems associated with the prior art layer-by-layerdeposition methods (eg., ALE) are overcome.

Other objects, advantages and salient features of the invention willbecome apparent from the following detailed description, which, taken inconjunction with the annexed drawings, discloses preferred embodimentsof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the apparatus for performing the subjectmethod.

FIG. 2 shows a high resolution isothermal desorption spectrum and, theinset, shows reflectivity change v. time for adsorption layer formation.

FIG. 3 shows a low resolution isothermal desorption spectrum produced inaccordance with the subject invention.

FIG. 4 is a graphical representation of reflectivity v. time for filmthickness in heteroepitaxial growth.

DETAILED DESCRIPTION OF THE INVENTION

The instant method and apparatus relates to the deposition of thin filmmaterials including metals in discrete submonolayer increments,insulators, superconductors, semiconductors and alloys thereof. Themethodand apparatus are especially useful for the deposition of thinfilms of covalently bonded materials and semiconductor films selectedfrom elementsof Group IV of the Periodic Table. The high degree ofthickness control provided by the subject method and apparatus is usefulin the growth of artificially structured materials. Additionally, thesubject invention could be useful in the development of electronicdevices based upon bandgap engineering.

Prior to discussing the specifics of the subject method and apparatus itisimportant to note that molecular sources used in thin film growthoften generate products other than just the thin film material. Theseside products remain on the growth surface and hinder thin film growthby occupying active sites necessary for the formation of the next layerof a thin film. For example, chemisorption of digermane used ingermanium growth occurs by adsorption of hydridic digermane fragments.The hydrogen is a side product that prevents further chemisorption ofdigermane. Consequently, in order for a surface chemical reaction to beviable for thin film growth, desorption of the side products must beefficiently controlled, while providing for incorporation of the desiredspecies into the surface layer to produce a thin film. In the subjectinvention, desorption and adsorption on the growth surface are monitoredin-situ to assess the active site density on the growth surface.

As to the following discussion, the term "chemisorption" refers toadsorption of digermane molecules on a substrate surface in the form ofhydridic germanium fragments. The term "adsorption layer" refers to thehydridic fragment layer. The term "growth surface" refers to any surfaceof the substrate, thin film or adsorption layer formed on the substratehaving surface sights (blocked or unblocked) for the formation of a thinfilm layer.

In studying the subject invention, it should be understood that the thinfilm growth process resulting from the adsorption layer formation iscorrelated with the isothermal desorption spectrum (`ITDS`) of thegrowth surface. The ITDS provides real time in-situ information aboutthe processes occurring on the growth surface. Furthermore, theconcentration of active sites produced by desorption of surface reactionproducts are derived from the ITDS. As a result, the ITDS is utilized inthe subject invention to control thin film growth in a digital ordiscrete submonolayer increments, i.e., digital epitaxy.

The instant specification demonstrates the use of the ITDS for epitaxialthin film growth of germanium from digermane source molecules. When adigermane pulse hits the heated substrate surface, some of the moleculesstick to it by occupying active surface sites to form an adsorptionlayer.The adsorption layer formed by the digermane has side products,hydrogen, that must be removed by way of desorption before another layermay be applied to the substrate. Monitoring of the adsorption anddesorption provides a closed loop control system for the subjectinvention. However, the control mechanism described for germanium growthis quite general and is applicable for thin film growth of any othermaterials within the spirit of the subject invention.

The preferred embodiment of the apparatus for performing the instantinvention is shown in FIG. 1. The substrate 2 (eg., Si(100)) ispositionedin a 23 liter stainless steel vacuum chamber 4 that iscontinuously pumped by a high throughput, high pumping speed, vacuumpump 6 (eg., a 170 liter/sec turbo-molecular pump, backed by a two-stagemechanical pump). The substrate 2 is attached by quartz clips (notshown) to a substrate manipulator 8, that is used for adjusting thesubstrate 2 position with respect to a pulsed molecular beam valve 12and a laser beam 23. Additionally, the substrate 2 is heated by apyrolytic graphite filament heater 10 encapsulated inpyrolytic-boron-nitride and the temperature of the substrate ismonitored by an infrared radiation thermometer (not shown), althoughother heating system may be used.

The pressure in the chamber prior to film growth is typically in the low10⁻⁷ Torr region. Film growth is initiated by short gas pulses(e.g.,pulses of digermane) directed at the surface of a heated substrate2. The gas pulses are delivered to the substrate by a fast acting,pulsed molecular beam valve 12 which is connected to a source gasreservoir 14 bya gas handling manifold 16 having safety related systems.Additionally, thepulsed molecular beam valve 12 is attached to a lineartranslator 18 used to change the distance between the pulsed valve andthe substrate. The pulsed molecular beam valve 12 is preferably asolenoid operated valve constructed in a conventional manner to performwithin the spirit of the subject invention.

The pulse duration, which is adjustable electronically, is typically afew milliseconds. The pulsed molecular beam valve 12 is shielded fromthe heated substrate 2 by a water cooled housing (not shown). The peakpressure in the chamber 4 during the pulse on-time never exceeds10⁻³Torr. The time-averaged pressure during growth depends on the pulserepetition rate: typically in the mid 10⁻⁴ Torr region at 3 Hz.

As previously discussed, a preferred embodiment of the inventionutilizes digermane for germanium growth. As such, short gas pulses ofdigermane areproduced by the pulsed molecular beam valve 12. The pulsesof digermane aredirected at the substrate 2 and the layer growth occursby adsorption of the hydridic digermane fragments at the active sightsof the substrate. Hydrogen is a side product that blocks active sightson the adsorption layer of digermane and prevents further chemisorptionof digermane.

For example, a 5% mixture of digermane in helium was used for digitalepitaxial germanium growth on Si(100). An expansion from the source gasreservoir, with stagnation pressure in the range of 20-50 psia, througha nozzle with an orifice diameter of 0.150 mm produces a supersonic gasjet.However, digital epitaxy is not limited to using only supersonicjets. Subsonic jets of pure source gases are also capable of producingsaturation surface coverage from short duration pulses.

In order for a surface chemical reaction to be viable for thin filmgrowth,desorption of the side products must be effectively andefficiently controlled, while providing for incorporation of the desiredspecies into the surface layer to produce a thin film.

The formation and evolution of the adsorption layer following the gaspulseis monitored by studying the isothermal desorption spectrum of thegrowth surface. The ITDS is measured by time-resolved reflectometry. Inthe preferred embodiment, a high stability (amplitude noise 0.02% rms)helium-neon (632.8 nm) laser 20 is reflected from the surface of thesubstrate and detected by a Si photodiode 22. Specifically, the laser 20directs a beam of light 23 at a mirror 24 which reflects the lightthrougha first window 26 in the chamber 4 and toward the substrate 2.The light beam 23 is reflected by the substrate 2 to produce a resultantlight signal 28 which exits the chamber 4 through second window 30. Thesignal 28 then passes through a narrow bandpass filter 32 and isdetected by the photodiode 22.

The signal is then digitized by an analog to digital converter 34(`ADC`) and stored in a microcomputer 36. The timing between opening ofthe pulsedvalve and the onset of data collection is adjusted by anelectronic delay generator 38.

The adsorption layer formation and evolution manifests itself inmodulationof the intensity of the specularly reflected light from thelaser. After detecting the signal 28 from the growth surface by thephotodiode 22, it is collected, displayed, and stored by a microcomputer36. An oscilloscope(not shown) can also be used for displaying thetransient signal.

As stated above, the formation of the adsorption layer is monitored inrealtime by time resolved reflectometry. The reflectivity changecorresponding to a digermane pulse is referred to as the transientreflectivity signal. The transient reflectivity signal shown in the highresolution ITDS of FIG. 2 consists of a steeply falling edge thatcorresponds to the adsorption layer formation. Investigations show thatdigermane chemisorption is rapid and independent of substratetemperature. The steepfall illustrates rapid chemisorption. The recoveryof the transient reflectivity signal past the maximum amplituderepresents the isothermal desorption spectrum. The amplitude of the ITDSis directly proportional tothe density of blocked sites (see FIGS. 2 and3). The amplitude of the ITDSis at its maximum when all available activesites are occupied. The time dependent recovery of the ITDS amplitude isassociated with regeneration of the active sites.

Since digermane chemisorption is rapid and independent of the substratetemperature, very short digermane pulses (2-3 milliseconds) are used tosaturate all the active surface sites. Once all the active sites areoccupied, no additional digermane molecules can bond to the substrate bychemisorption. The chamber is continuously pumped to evacuate theunreacted portion of the digermane pulse from the vacuum chamber toproduce exactly a single uniform monolayer coverage.

Before net film growth can occur by the application of a seconddigermane pulse, the active sites must be regenerated by eliminatingonly the side products that occupy the active sites, in this casehydrogen. In the instant invention, the active surface sites areregenerated by molecular hydrogen desorption induced by substrateheating. That is, the substrate temperature is fixed such that theevacuation of the unreacted digermane is much faster than hydrogendesorption. As a result, the germanium atoms from digermane remain onthe surface of the substrate becoming part of thefilm being created andthe unreacted digermane is pumped away from the substrate, while thehydrogen is removed from the digermane by desorption.Ideally, thisprocess adds one monolayer of germanium to the film.

Since no digermane is present in the chamber during the time between twosuccessive pulses (pulse period), hydrogen desorption fully regeneratesthe active layer in the time between the two successive pulses. Afterthe active surface is regenerated, a new digermane pulse is used toinitiated growth of the next monolayer of film.

As previously discussed, time-resolved reflectometry is used to monitortheresidual hydrogen coverage. The use of reflectometry is based onchanges inthe growth surface electronic structure in response toadsorption of hydridic digermane fragments. A detailed description ofthe principles of optical reflectometry and its application is given ina paper by J. D. E. Mcintyre and D. E. Aspnes, Surf. Sci. 24, 417(1971). In the present technique an ultra stable HeNe laser is reflectedfrom the surface of the growing film. Chemisorption of digermaneproduces a decrease in the intensity of the specularly reflected lightthat is linearly proportional to the surface coverage. Regeneration ofthe active sites by hydrogen desorption recovers the originalreflectivity signal. Therefore, the transient reflectivity signal can beused to monitor the kinetics of the adsorption layer formation and thehydrogen desorption.

More specifically, thin film growth correlates with the amplitude of thetransient reflectivity signal in the following manner. At the maximumamplitude the density of the active film growth sites is lowest becauseall the sites are occupied by hydrogen that prevents chemisorption ofdigermane. Consequently, the maximum of the ITDS amplitude correspondsto zero film growth. Recovery of the transient reflectivity signalindicates that active sites are being regenerated and that chemisorptionof new digermane molecules can occur. FIG. 2 shows a high resolutionITDS of digermane on Si(100). The kinetic order of the desorptionprocess determines the behavior of the ITDS. The simplest desorptionprocess is described by a single exponential first order reaction. Theactive sites for germanium growth are regenerated by such a first orderhydrogen desorption process. The hydrogen desorption process was foundto be strongly temperature dependent with an activation energy of E=1.7eV and afrequency factor of 1.6×10¹³ sec⁻¹.

In summary, the instant invention utilizes the hydrogen desorptionprocess,which is relatively slow at low and intermediate substratetemperatures (much slower than digermane chemisorption), in combinationwith pulsed digermane delivery as an externally controlled self-limitingfilm growth mechanism. Hydrogen coverage of the growth surface producedby chemisorption of hydridic digermane fragments terminates furtherchemisorption until the active sites are regenerated by hydrogendesorption. The active sites are regenerated after the digermane hasbeen evacuated from the vacuum chamber to prevent uncontrollablechemisorption from the background. The slow hydrogen desorption processat low substratetemperatures prevents successive chemisorption ofmultiple digermane molecules at a single active site and allowsevacuation of all the unreacted digermane following the digermane pulse.As a result, without desorption of the hydrogen side product, thechemisorption process of digermane is limited to a monolayer coverage.Consequently, only those active sights that are regenerated during thedelay time between pulses (i.e., pulse period) can chemisorb new sourcegas molecules from the next pulse.

The corresponding hydrogen coverage oscillations on a surface arerelated to germanium film growth rates by the following equation:

    G=(a.sub.0 ÷4)(1÷t)Θ.sub.0 [1-exp(-k.sub.d t)].

Where G (nm/s) represents the growth rate, a₀ is the lattice constant ofgermanium, and t(s) is the pulse period. Θ₀ is the hydrogen surfacecoverage at the end of the pulse. Θ₀ has a fixed value between 0 and 1depending on the experimental conditions. k_(d) (s⁻¹) is the hydrogendesorption rate constant. The growth rate G =(a₀ /4)Θ₀ k_(d) forcontinuous digermane supply is obtained as a limiting case of the aboveequation for infinitely small (t→0) pulse periods.

For a particular substrate the amplitude of the hydrogen coverageoscillations depends on the substrate temperature, the source gas pulseperiod, and the pump-out time constant. The interference caused bychemisorption of background digermane can be reduced or eliminated byincreasing the pumping speed of the apparatus. For a particularmaterial, the amplitude of the oscillations depends on thecrystallographic orientation of the substrate. Other experimentalparameters may also influence the amplitude of the oscillations. Inaddition to Ge(100), hydrogen coverage oscillations are also observed onGaAs(100), Si(111), and Si(100). The oscillations are produced on aSi(100) surface at 743 K substrate temperature from a burst of digermanepulses with a pulse periodof 0.333 s. Stopping the pulsation results indepletion of the surface.

Regeneration of the active surface sites is monitored and after all theactive sites are regenerated, a new digermane pulse is used to initiatedthe growth of the next layer. Alternatively, partially dehydrogenatedsurface layers can be utilized to control the growth process at thesubmonolayer level.

The closed-loop feedback mechanism of the instant invention is achievedby monitoring the ITDS and, consequently, the surface coverage of thesubstrate via the transient reflectivity signal. In-situ real timemonitoring of the ITDS generates data from which corrections to thesubstrate temperature, pulse intensity, and delay time between pulsescan be made during the growth process to maintain the film growthprocess within the desired parameter space. The film thickness depositedper pulsedepends on the number of active surface sites that areavailable when the source gas pulse arrives. FIG. 3 illustrates theimportance of the real time feedback mechanism for obtaining the correcttiming between successive source gas pulses. The density of active sitesregenerated at afixed substrate temperature depends on the pulse period.For short pulse periods the low density of regenerated active sites (asmall change in thetransient amplitudes) results in poor control overthe film growth process.Optimal conditions for monolayer control of thefilm growth process are achieved by adjusting the pulse period and thesubstrate temperature such that digermane chemisorption is terminated atmonolayer coverage and the sites blocked by hydrogen coverage are fullyregenerated by desorption before arrival of a new digermane pulse.

The subject invention is contemplated for use for the growth ofepitaxial thin films consisting of 1) elemental material, 2)superlattices consisting of alternating layers of two or more differentmaterials and 3)ultrathin binary alloy layers.

Epitaxial growth of single component materials is demonstrated forgermanium growth on Si(100) substrates from a multitude of discretedigermane pulses. FIG. 3 illustrates a typical oscillatory responseobtained from monitoring the growth process. The oscillations correspondto hydrogen coverage changes from discrete digermane pulses that produceaper pulse increase in the net film thickness. The net film thicknesscan beobtained by simply counting the coverage oscillations.

Growth of superlattices consisting of alternating layers of two or morematerials is a simple extension to using multiple sources. For example,Si/Ge superlattices can be grown by using alternating single pulses orpulse sequences (depending on the desired film thickness) of disilaneand digermane.

Ultrathin binary alloys are grown by periodically introducing a singlepulse or burst of pulses of one component at high intensity into acontinuous low pressure growth environment of the component. Forexample, a roughly 90% Ge containing thin alloy layer can be grown byintroducing adigermane pulse with a flux that is 9 times higher than theflux from a lowpressure steady-state background growth of silicon fromdisilane.

Digital epitaxy, as disclosed by the subject invention, can be used forthedeposition of any material that occurs by adsorption layer formationand thermal regeneration of active adsorption sites. The surfacecoverage during film growth is governed by intrinsic factors andcharacteristics ofthe material involved. Additionally, surface coverageduring film growth isgoverned by extrinsic factors such as substratetemperature, intensity of source gas pulse, and beam modulationfrequency.

The controlling mechanism utilized in digital epitaxy is based on thecombined effect of slow hydrogen desorption and rapid digermanechemisorption. The temperature independent rapid chemisorption allowssaturation of the active sites from a short duration digermane pulse.The digermane pulses are only 2 to 3 milliseconds long compared toseconds or minutes of exposure in ALE. The substrate temperature ismaintained such that the temperature dependent hydrogen desorptionprocess is much slower than the chamber pump-out time (i.e., theevacuation time for the gas pulse). This requirement limitschemisorption of digermane to one moleculeper site and allows evacuationof the unreacted portion of the gas pulse. In-situ real-time monitoringof the growth surface is used to determine the growth parameters thatsatisfy these requirements. Many other surface reactions exhibit similarbehavior, that is, a rapid chemisorption step followed by a temperaturedependent desorption of products, and could be incorporated within thespirit of the subject invention.

The rapid initial chemisorption step can be terminated at a monolayerthickness by adjusting the substrate temperature such that theregeneration of the active sites is negligible during the pump-out timeofthe unreacted portion of the gas pulse. Thin film growth occurs byregenerating the active sites in the absence of the source gas andsubsequent chemisorption of digermane molecules provided by a new sourcegas pulse. Compared to ALE, the subject invention requires only onesourcegas to realize thin film growth. Typical thickness per pulsevalues of 0.4 monolayers were observe for germanium growth on Si(100).If the delay timebetween successive pulses is too short, most of thesource gas will scatteroff without contributing to film growth. Theoptimum pulse repetition rate is determined by the characteristics ofthe surface reaction, and derived from the ITDS. For example, testresults show that where the repetition rate is 3 Hz, the average growthper pulse was 0.25 monolayers, and, consequently, a growth rate of 0.75monolayers per second was obtained.

The closed-loop feedback system is achieved by monitoring the ITDS. TheITDS is used for determining the availability of active sites on thegrowth surface. The ITDS can be used to elucidate the type of surfacereaction (kinetic reaction order) operating at particular growthconditions. The information from the ITDS is used to generate thefeedbackloop. For example, digermane is provided only when the ITDSshows that active sites are available. The ITDS can also be used forin-situ tailoring of the composition of binary alloys of silicon andgermanium. For example, the growth of 50% silicon containing Si/Gealloys is achievedby timing the arrival of the silicon source gas at thestage when half of the surface sites are available for chemisorption ofthe silicon source (the other half was covered with germanium fragmentsfrom a preceding digermane pulse). Adjustments to surface temperature,source gas pulse intensity, and pulse timing can be made simultaneouslyin order to maintain the self-limiting growth mode.

Further, ultrathin films of uniform thickness can be achieved by thesubject method and apparatus. The controlling mechanism provides themeansfor precise external thickness control. The growth-per-pulse can bedetermined by monitoring the amplitude of the signal produced by theadsorption layer resulting from each source gas pulse.

Additionally, improved efficiency of growth from superthermal sourcemolecules is achieved by the subject invention. As stated above, amixtureof 5% digermane and helium has been tested for germanium growthin accordance with the subject invention. The digermane moleculesexpanded from a high pressure nozzle acquire superthermal energies.These energeticmolecules improve the conversion efficiency of the sourcegas into thin film material by increasing the reaction efficiency of theincident sourcemolecules at the growth surface. However, the inventionis not limited to use with supersonic expansions. Pure source gases aswell as their low concentration mixtures with inert carrier gases can beused in digital epitaxy.

As stated previously, the invention is not limited to germanium thinfilm growth, nor to covalent semiconductor thin film growth. Thescientific principles involved in determining and maintaining a surfacecoverage of adsorbates are quite general. Consequently, this method andapparatus can be used for thin film growth of other materials such asmetals, insulators, superconductors, semiconductors, and for alloys ofthese materials. The technique is most suitable for growth of artificialstructures consisting of multi-layers of various materials, but it canbe used for growth of thicker materials of high surface uniformity. Thehigh degree of thickness control can be utilized for growth ofartificially structured materials consisting of thin layers ofsemiconductors, ceramics, metals, and superconductors. Since this is theonly technique available to date for closed-loop feedback controlledlayer-by-layer growth of covalently bonded Group IV semiconductors it isof major significance in the implementation of three dimensionalintegration for production of silicon integrated circuits of highcomplexity.

EXAMPLE

The following is exemplary of the method and equipment which may be usedfor the formation of an adsorption layer within the spirit of theinstant invention.

As to the pulsed gas, it is released through an orifice at pressuressufficiently high that the flow in the vicinity of the orifice isgovernedby hydrodynamic equations and results in the gas obtainingsupersonic velocities. Quantitively, the criterion for obtainingsupersonic flow is that the mean free path (mfp) for molecule-moleculecollisions must be much less than the dimension that characterizes theorifice. For a circular opening this dimension is the diameter, d. Thesupersonic jet flow used in accordance with the subject invention, withmfp<<d, is most easily viewed as one of the three possible flow regimesfor an expansion through a nozzle, the other two being effusive flow(mfp>>d) and intermediate flow (mfp≈d).

The pulsed molecular beam valve is solenoid operated. More specifically,a fast rise-time power supply (not shown) drives a current(approximately 15amps) through the coil in the valve to break the sealbetween the plunger and the O-ring. This controls the flow of gas fromthe reservoir through the orifice. The thickness of the orifice shouldbe less than the diameterof the orifice in order to suppress both theformation of clusters and the concentration of gas along the expansionaxis.

The gases are binary mixtures of digermane in an inert carrier gas,while the fluid dynamic description of the expansion utilizes a singlevelocity that depends on the mean molecular mass. In the idealhigh-density expansion both gases will obtain the same velocity. Inpractice, deviations from velocity equalization are caused by theinefficiency of kinetic energy exchange between molecules of unlike massand can be analyzed in terms of a parameter called the velocity slip.

For an expansion at absolute temperature T and with negligible slippage,a component of mass M in a mixture with mole-fraction weighted averagemass <m> attains a mean kinetic energy of 2.5 k_(B) T(M/<m>). The dataavailable on this entrainment phenomenon suggests that our expansionconditions meet the low slippage requirement. If this is actually thecase, then mixtures of 5%Ge₂ H₆ /He and 5%Ge₂ H₆ /Ar provide thedigermane molecules with kinetic energies of≈0.8 eV and ≈0.2 eV,respectively, assuming k_(B) T=0.026 eV. These energies areapproximately 8 to 30 times greater than a typical thermal energy.Reactions between small, saturated molecules and clean metal surfacesare frequently found to be greatly accelerated by such superthermalenergies. In sharp contrast, test results indicate little if anydifference in semiconductor film growth efficiencies for the twomixtures just mentioned.

In this example, the pulsed molecular beam valve rapidly deposits anadlayer on the growth (substrate or film) surface, allowing us tomonitor its subsequent evolution. The angular distribution of the growthmolecules(i.e., the flux of digermane as a function of the angle α) istherefore not essential information. It is useful, though, to makeestimates of the flux based on the most pertinent available data andtheory. As a matter of convenience, supersonic jet flux distributionsoften are characterized by a (cosα)^(m) distribution. Drawing onpublished data, we expect the angular distribution of digermanegenerated by our expansion conditions to have 3<m<6. Applying thisdistribution, theinstantaneous (time-averaged) intensity, I, iscalculated from the instantaneous (time-averaged) flow rate, F, as

    I(R, α)=F(m+1)(cosα).sup.m /(2πR.sup.2) .

I(R,α) is the fluence, i.e., the number of molecules per unit area perunit time crossing a sphere of radius R centered on the nozzle. Foranelement of area at a distance R from the orifice and located on aplane that is perpendicular to the expansion axis and a distance D fromthe orifice, this can be rewritten

    I(D,α)=F(m+1)(cosα).sup.m+3 / (2πD.sup.2).

Two of the additional three powers of the cosine come from D=Rcosα; thethird arises from the scalar product of the molecular velocity vectorwith the surface normal.

The substrates are ≈2.4×2.4 cm² pieces cut from single-crystal wafers ofeither (100) or (111) orientation. After being degreased with solventsthe Si substrates are etched according to the RCA recipe¹⁴ (15 minuteseach in 5:1:1 H₂ O/H₂ O₂ /NH₄ OH and 6:1:1 H₂ O/H₂ O₂ /HCl solutions,both at 80° C.) and dipped in dilute HF for 45 seconds. After a final H₂O rinse the substrates are blown dry with N₂, mounted and placed in thegrowth chamber. The germanium wafers used receive the same treatmentminus the RCA clean. Studies with Si indicate that, used in conjunction,the RCA clean and HF dip remove nearly all of the surface oxide andcarbon. In addition, infrared spectroscopy shows that the resultingsurface is primarily terminated by a passivating hydride.

Further preparation carried out in-situ just prior to film growth varieswith the type of substrate. All experiments involving Si substratesinclude the growth of 25 nm of Ge at 450° C., regardless of thetemperature used for further deposition. This practice enhanced thegrowthrate when subsequent deposition occurs at higher temperatures.Prior to this, the Si substrates are either baked at 800° C. for 5minutes or they are simply heated slowly to 450° C. while being exposedto a very low flux (≈2×10¹⁴ /cm² -s) of the growth gas.

A high-temperature bake is standard for the preparation of a clean,reconstructed Si surface in ultra-high vacuum. In our vacuum, though,the rate of surface oxidation is probably so great that it is impossibleto maintain a clean surface during the cooldown to 450° C. Moreover, itis not definite that a bake temperature of 800° C. is adequate to cleanthe surface in the presence of ≈3×10⁻⁷ Tort ofwater vapor. For thesereasons, growth was initiated as the protective hydride was desorbed byintroducing digermane during heating of the substrate.

For Ge substrates the in-situ preparation consists simply of a bake at700° C. for 5 minutes. Because GeO desorbs from a Ge surface attemperatures above 600° C., this bake temperature is quite adequatetoremove the predominant contaminants.

The chamber pressure is kept low in order to minimize scattering of thejetmolecules by background gas. Accordingly, the pulsed valve isadjusted so that, with a reservoir pressure of 1250 torr and arepetition rate, RR, of3 Hz, the time-averaged chamber pressure isnominally 0.5 mTorr. All experiments discussed here had a pulseduration, T_(p), of ≈2.9 msec.

To quantify the gas flow rate, at the end of each experiment a knownnumberof pulses are released into the closed chamber and the pressurerise is recorded. From this measurement of the number of moleculesreleased per pulse, N_(p) ≈6×10¹⁷, the time-averaged, N_(p) ×RR, andinstantaneous, N_(p) /T_(p), flow rates are obtained. Using thementioned values for RR and T_(p) they are found to be nominally1.8×10¹⁸ molecules/sec and 2.1×10²⁰ molecules/sec, respectively. For a5% Ge₂ H₆ /95% He mixture, with m=3, and D=4.1 cm, the correspondingmolecular fluences at the growthsurface (for α=0) are ≈3.6×10¹⁵ /cm² -s(time-averaged) and ≈4.0×10¹⁷ /cm² -s (instantaneous). Note that thenominal dose per 3 msec pulse is 1.2×10¹⁵ /cm², equivalent toapproximately two monolayer exposure for (100) Si or Ge surfaces.

The background gas flux on the growing surface also should beconsidered. The most useful gauge of this flux contribution is thedosage per pulse repetition period. The four experimental parametersthat controlled this dosage were N_(p), RR, the chamber volume, and thepumping time constant τ. We have independently determined that τ=0.25sec, and the othernecessary information has been given above. Using aneffective (i.e., thermally and directionally averaged) digermanevelocity of 1.3×10⁴ cm/sec, we calculate that 3×10¹⁵ /cm² backgroundmolecules strike the surface between pulses. According totheseestimates, then, only about 30% of the time-averaged flux on thegrowth surface was due to the jet. The instantaneous jet flux, though,was at least 30 times greater than the peak background flux.

These gas flow parameters were held fixed in order to study the growthrates as a function of substrate temperature. Slightly different testswere performed specifically to study the adsorption layer decompositionprocess(es). In these tests the repetition rate was optimized to allowfull surface regeneration at each temperature.

A delay generator is used to trigger both the gathering of data by thecomputer and (a few milliseconds later) the opening of the pulsed valve.For each gas pulse, a reflectivity reading is made just prior to itsarrival at the surface and another is made just as the pulse ended. Inthis manner a record of the pulse-induced reflectivity change as thefilm thickness increased was obtained. The temporal evolution of thereflectivity transient is recorded by monitoring the reflectivitythroughout the course of the pulse/pumpout cycle.

A single pulse normally changes the sample reflectivity by ≈0.3% whichis approximately a factor of six above the background noise level. Ininvestigation of the surface properties enough pulses are averaged inorder to reduce the noise level to ≈5% or less of the transient signal.However, the signal from a single pulse is sufficient for processcontrol in practical application.

After averaging and smoothing, we obtain signals such as that displayedin FIG. 2. Choosing the average value of the first few milliseconds (theprepulse reflectivity) as the baseline, R₀, and writing subsequentreadings as R(t), we define the normalized differential reflectivity as##EQU1##and refer only to this quantity when discussing the transientsignals. For heteroepitaxial growth the reference signal, R₀, variesslowly and isused to monitor the film thickness in-situ, as shown inFIG. 4.

While advantageous embodiments have been chosen to illustrate theinvention, it will be understood by those skilled in the art thatvarious changes and modifications can be made therein without departingfrom the scope of the invention as defined in the appended claims.

We claim:
 1. A method for the formation of thin film materials, comprising the steps of:(a) delivering a pulse of gaseous material to a growth surface of a substrate to form an adsorption layer of said gaseous material on said growth surface; (b) monitoring available active sites on said growth surface and controlling the delivery of said gaseous material according to the availability of active sites on said growth surface; (c) growing an epitaxial layer on said growth surface.
 2. A method according to claim 1, wherein step (b) further includes the steps of:monitoring available active sites on said growth surface; evacuating said gaseous material from the area adjacent said substrate after a saturated adsorption layer of said gaseous material has formed on said substrate; exposing said growth surface to a second pulse of gaseous material when a concentration of active sites are available on said growth surface; and repeating said steps of monitoring, evacuating, and exposing until a thickness is achieved.
 3. A method according to claim 2, further including the step of heating said substrate to facilitate regeneration of active sites on said growth surface.
 4. A method according to claim 2, wherein monitoring is achieved by time-resolved reflectometry.
 5. A method according to claim 4, wherein an isothermal desorption spectrum of said growth surface is studied to determine the availability of active sites.
 6. A method according to claim 1, wherein an isothermal desorption spectrum of said growth surface is studied to determine the availability of active sites.
 7. A method according to claim 6, wherein said isothermal desorption spectrum amplitude is directly proportional to the availability of active sites.
 8. A method according to claim 1, wherein said pulse of gaseous material is digermane and step (b) further includes the steps of:monitoring adsorption of said digermane pulse; evacuating said digermane pulse from the area adjacent said substrate when adsorption is complete; monitoring desorption of hydrogen from said growth surface; exposing said growth surface to a second pulse of digermane when said desorption of hydrogen is complete; and repeating the steps of monitoring adsorption, evacuating, monitoring desorption, and exposing until a desired thickness is achieved.
 9. A method according to claim 8, further including the step of heating said substrate to facilitate desorption of hydrogen from said growth surface.
 10. A method according to claim 8, wherein monitoring is achieved by time-resolved reflectometry.
 11. A method according to claim 10, wherein an isothermal desorption spectrum of said growth surface is studied to determine the availability of active sites.
 12. A method according to claim 8, wherein an isothermal desorption spectrum of said growth surface is studied to determine the availability of active sites.
 13. A method according to claim 12, wherein said isothermal desorption spectrum amplitude is directly proportional to the availability of active sites. 